The customer is usually seated. It's at the wish of the guest. This service may either have buffet or menu. If there is buffet then customer have to help them and if there is a menu then staff will take care of service and clearance both.
Commonly applied to Carry "type operations, where some of the part of the meal is served and then rest is to be helped or collected by the guests themselves.
Buffet where guests select their own Food and Beverages and help themselves.